1. Field of the Invention
The present invention relates to a method of compensating for biases for effective servo control of a hard disc drive, and more particularly, to a method of measuring a bias compensation value for short track seeking of a hard disc drive and a method of compensating for a bias using the same.
2. Description of Related Art
In general, a hard disc drive, which magnetically records data or reproduces data from a rotating magnetic disc, is capable of accessing a large amount of data at a high speed. Thus, the hard disc drive has been frequently used as an auxiliary storage device of a computer system.
In the hard disc drive, data is stored in tracks formed on a disc in concentric circles. For a read/write operation, such tracks are accessed by a magnetic head. The magnetic head moves over a magnetic disc in a radial direction so that it can be positioned on a desired track, which is referred to as track seeking.
For track seeking, disturbances acting on the magnetic head are referred to as a bias. The bias varies depending on several factors, including position, moving distance, and moving direction of the magnetic head. That is, during track seeking, initial driving power of a voice coil changes according to the position and moving direction of the magnetic head, and initial moving speed of the magnetic head changes according to the moving distance of the magnetic head. Accordingly, the bias must be compensated for based on the position, moving distance, and moving direction of the magnetic head.
A value for compensating for bias effects (hereinafter referred to as a “compensation value”) is measured during a bias calibration process performed when power is supplied to a hard disc drive. The measured bias compensation value is stored in a bias compensation table and used for track seeking or track following.
Conventionally, the bias compensation value is measured while moving the magnetic head in a specified direction of a disc, e.g., in an outer diameter direction, and then measured again while moving the head in another direction of the disc, e.g., in an inner diameter direction. In this case, a distance traveled by the magnetic head corresponds to 10,000 or more tracks.
Accordingly, when bias compensation curves are obtained by moving the magnetic head a long distance, e.g., moving it in the outer diameter direction or in the inner diameter direction, an offset between the bias compensation curves is very large.
It is assumed that short track seeking corresponding to less than 1000 tracks is performed while reversing direction of the track seeking. In this case, a bias caused is not large. Nevertheless, when the compensation curves for long track seeking are applied to compensate for the bias, the compensation curves cause disturbances to the magnetic head since bias compensating value is at least as large as the offset, thus increasing the time required to perform track seeking.
There has been an effort to increase the radial density of concentric data tracks in high performance hard disc drives, thus resulting in a reduction in track width. Therefore, it is very important to compensate for a bias during track seeking, not only when moving the magnetic head a large distance, e.g., 10000 tracks or more (hereinafter referred to as “long track seeking”) but also when moving it a short distance, e.g., 1000 tracks or less (hereinafter referred to as “short track seeking”).